The Cell and the Hierarchy of Life

• The atom is the smallest and most fundamental unit of matter.
• Atoms consist of a nucleus surrounded by electrons
• Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization.
• polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules)
• monomers are simpler than macromolecules
• Macromolecules can form aggregates within a cell that are surrounded by membranes; these are called organelles.
• Organelles are small structures that exist within cells.
• Cells are classified as prokaryotic or eukaryotic.
• Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei.
• cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus
• In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions.
• Organs are collections of tissues grouped together performing a common function.
• An organ system is a higher level of organization that consists of functionally related organs.
• Single-celled prokaryotes and single celled eukaryotes are also considered organisms and are typically referred to as microorganisms.